Although I havn't read all the ins and outs of this deal. I cant see a problem. You pay just over £20 with a return of £45. I have read that Quidco are quite good with paying cashback with Vodaphone.
If there are any queries or problems, please let me know.
This should also work with the other sim only deals:
£10 sim only = £15 cashback
£15 sim only = £25 cashback
£20 sim only = £45 cashback